Kenneth Olden(http://www.niehs.nih.gov/about/od/pastdirectors/kennetholden.cfm), Ph.D., Director of the National Institute of Environmental Health Sciences(http://www.niehs.nih.gov) and the National Toxicology Program, which has the responsibility for preparation of this report, said that publication of this addendum follows the recent ruling by the U.S. Court of Appeals for the District of Columbia Circuit dismissing a request for an injunction to prevent the listing of TCDD as a "known human carcinogen" pending appeal of the district court's decision upholding the listing.

The change in the listing of TCDD from the "reasonably anticipated" to the "known to be a human carcinogen" category had been planned to occur in the Ninth Report, but the designation was delayed by litigation. The plaintiffs in the lawsuit claim that the Department of Health and Human Services(http://www.hhs.gov) and National Toxicology Program(http://ntp.niehs.nih.gov/) had improperly applied the Report on Carcinogens' listing criteria in listing TCDD as a "known human carcinogen." The Ninth Report was published last May 15 with TCDD listed as a "reasonably anticipated" human carcinogen but with a statement included indicating an addendum may be published following the Court's ruling on the litigation.

The National Toxicology Program's listing of TCDD in the " known" category is based on sufficient evidence of carcinogenicity from studies in humans, involving a combination of epidemiological and mechanistic information which indicates a causal relationship between exposure to TCDD and human cancer.

TCDD is not deliberately produced today but has been found as a contaminant in some herbicides and pesticides and is formed as an inadvertent by-product of incineration of waste. TCDD levels in Americans have declined in recent years as a result of environmental controls but is still widely detected in the environment and can be found in very small amounts in the general population. The Report on Carcinogens is a cancer health hazard identification document that discusses substances that may pose a carcinogenic hazard to human health. The report does not present quantitative assessments of carcinogenic risk, an assessment that defines the conditions under which the hazard may be unacceptable.

The listing of substances in the report, therefore, does not establish that such substances present carcinogenic risks to individuals in their daily lives. Such formal risk assessments are the purview of the appropriate Federal, State, and local health regulatory and research agencies. Both the Environmental Protection Agency(http://www.epa.gov) and the Food and Drug Administration(http://www.fda.gov) have quantitative assessments of dioxin's cancer risks.

The Report on Carcinogens is a Congressionally mandated listing of known and reasonably anticipated human carcinogens. Its preparation is delegated to the National Toxicology Program, which is headquartered at the National Institute of Environmental Health Sciences, by the Secretary, Department of Health and Human Services. Section 301(b)(4) of the Public Health Service Act, as amended, provides that the Secretary, Department of Health and Human Services(http://www.hhs.gov) shall publish a report and also states that the reports should provide available information on the nature of exposures, the estimated number of persons exposed and the extent to which the implementation of Federal regulations decreases the risk to public health from exposure to these chemicals. The revised Ninth Report that contains all addendum materials is available on the Internet from the National Toxicology Program's web page at Report on Carcinogens.

NIEHS supports research to understand the effects of the environment on human health and is part of NIH. For more information on environmental health topics, visit http://www.niehs.nih.gov (http://www.niehs.nih.gov/index.cfm) . Subscribe to one or more of the NIEHS news lists ( http://www.niehs.nih.gov/news/newslist/index.cfm (http://www.niehs.nih.gov/news/newsroom/newslist/index.cfm) ) to stay current on NIEHS news, press releases, grant opportunities, training, events, and publications.

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov (http://www.nih.gov/).